In petroleum refineries, hydrogen is used for hydrotreating and hydrocracking operations for the production of low sulfur transportation fuels. Recent regulatory push toward low-sulfur gasoline and ultra-low sulfur diesel products has necessitated refineries to upgrade and expand desulfurization capacity. Desulfurization is primarily accomplished by hydrotreating high-sulfur refinery streams. Severe hydrotreating, requiring significant consumption of hydrogen, is necessary to reach the very low levels of sulfur being required, 30 ppm for gasoline and 15 ppm for diesel fuel. This drive toward cleaner fuels is anticipated to continue into the next decade, resulting in further increases in demand for hydrogen.
Currently, US hydrogen production is about 9 million tons per year of which 85% is used at the site of generation. About 95% of this hydrogen is made by steam reforming of natural gas. As a result, the cost of hydrogen is highly sensitive to natural gas prices. Recent spot prices for natural gas have been volatile, ranging between $6 and $14 per million Btu and have averaged around $10 per million Btu during 2005. Thus the need for alternate options that minimize exposure of hydrogen prices to volatile natural gas market continues to exist.
Hydrogen from gasification of carbonaceous materials such as coal and petroleum coke is one of the technically attractive options. Coking capacity in the U.S. has risen by about 60% in the past decade. Traditionally, U.S. Gulf Coast refineries producing high-sulfur coke have sold their petroleum coke into overseas markets at roughly breakeven values of less than $5 per ton. If natural gas prices continue to remain high in the future, there will be opportunities to use low-cost petroleum coke and/or coal in place of natural gas to produce hydrogen through gasification. The petroleum coke could also be used to produce SNG as a natural gas supplement. Since SNG uses the same infrastructure as natural gas, SNG could be easily sold through the existing pipeline system.
The main barrier to making hydrogen from coal and/or petroleum coke gasification is the high capital investment. Another drawback of producing hydrogen by gasification is that petroleum refineries require hydrogen to be available 98+% of the time. This level of reliability of supply is generally not possible with current gasification technology because gasification systems with a single gasifier have been shown to have only 80-85% availability particularly in the first few years of operation. Adding a spare gasifier helps, but it increases the capital cost appreciably and the availability is still not as high as that achieved from conventional SMR where the availability is more than 98%.
Thus it is desirable to develop processes that maximize the reliability of hydrogen production through the gasification of cheaper carbonaceous fuels while minimizing the impact on energy efficiency and cost of production. The carbonaceous fuel is any solid or liquid or gaseous combustible organic material that can be used as a feedstock to a gasification process to produce syngas. This invention reveals novel concepts and methods for providing hydrogen at a high level of reliability from a gasification system by integrating it with SMR.
A. Gasification, Gas Cleanup and Acid Gas Removal
The process of gasifying carbonaceous material into syngas is generally known in the industry. In gasification process syngas is commonly produced from gaseous combustible fuels such as natural gas or associated gas, and liquid and solid combustible organic fuels, such as, coal, petroleum coke, wood, tar sand, shale oil, and municipal, agricultural or industrial waste. The gaseous or liquid or solid combustible organic fuels are reacted at high temperature in a refractory-lined vessel with air, enriched air or high purity oxygen in an oxygen deficient environment in the presence of steam which acts as temperature moderator. When syngas from the gasifier is used to produce hydrogen and SNG, use of high-purity oxygen (95+ mol %) is the preferred mode of operation.
Any of the numerous commercially available gasification technologies can be utilized, for example, fixed (or moving) bed, fluid bed or entrained flow. The fixed bed technology has been used commercially since at least the 1940's and the leading technology is the Lurgi technology most notably employed by Sasol in South Africa and Great Plains Synfuels plant in North Dakota, USA. These gasifiers have proven track record of reliable operation with low rank coals. The alternative fixed bed technology that has also been tested on petroleum coke and municipal and industrial waste is the British Gas/Lurgi (BGL) technology. Although this technology is one of the preferred gasification technologies for the present invention because of its high methane content in the syngas, the handling of fines and large amounts of tars and oils co-produced with the syngas could be problematic and costly.
Fluid bed gasification technologies such as KRW and UGAS have not been commercially operated on a scale large enough but could be used with the present invention.
Entrained flow gasification technologies include E-Gas—two stage slurry feed technology (ConocoPhillips), Texaco—single stage slurry feed technology (General Electric), and Shell—single stage dry feed technology (Shell). The General Electric (GE) and ConocoPhillips technologies have commercial operating experience on a variety of carbonaceous feedstock including coal and petroleum coke.
In the reaction zone of the gasification reactor, the contents will commonly reach temperature in the range of 1,700° F. to about 3,000° F., and more typically in the range of about 2,000° F. to about 2,800° F. Pressure will typically be in the range of about 14.7 psia (atmospheric) to about 1500 psia, and more typically in the range 300 psia to 1200 psia.
In a typical gasification process the synthesis gas will substantially comprise of hydrogen (H2), carbon monoxide (CO) and lesser quantities of methane, water, carbon dioxide (CO2), carbonyl sulfide (COS) and hydrogen sulfide (H2S). The syngas is commonly treated to remove or significantly reduce impurities such as H2S, COS and CO2 before being utilized in down stream processes. A number of acid gas removal (AGR) systems are commercially available. Selection of AGR system will depend on the degree of sulfur compounds and CO2 removal required, and by the operating pressure of the AGR system. Suitable commercial chemical and physical solvent-based absorption processes may include amine-based processes such as methyldiethanolamine (MDEA) or activated MDEA technologies and physical solvent-based technologies commercialized under the trade names of Selexol, Morphysorb, Rectisol, Ucarsol, Purisol, and Fluor Solvent.
B. Power and Steam (IGCC)
Electric power can be generated efficiently in integrated gasification combined cycle (IGCC) systems. For IGCC application, the syngas produced in the gasifier after heat recovery and appropriate cleanup is fired as a fuel to the gas turbine system that drives a generator to produce electric power. Hot turbine exhaust can be passed to a heat recovery steam generation (HRSG) system to produce high pressure steam which can be expanded through a steam turbine to drive another electric generator to produce additional power. Such IGCC systems, if economically justified, can be appropriately integrated with the air separation units (ASU) to send diluent nitrogen from the ASU to the gas turbine and optionally compressed air from the gas turbine compressor to the ASU according to established procedure known in the art.
C. SNG Conversion
Conversion of gasification produced syngas to pipeline quality synthetic or substitute natural gas (SNG) is an established technology. In the 1970's concerns over a potential shortage of natural gas fostered considerable interest in the production of SNG from coal. A number of large-scale projects were planned of these projects only one large-scale commercial plant—the Great Plains Synfuels Plant located near Beulah, N. Dak. was ever built. The increased availability of cheaper North American natural gas in the 1980s and 1990s ended interest in large-scale production of SNG from coal. However, small-scale SNG production from LPG and naphtha has found a niche market in Japan and elsewhere where they provide backup fuel for natural gas based power generation.
The Great Plains facility, which started SNG production in early 1980s, uses about 18,500 tpd of lignite coal in 14 moving bed type Lurgi Mark IV gasifiers to produce about 170 MMscfd of SNG. Including planned and unplanned outages, the average annual plant loading factor is typically about 90-92%. This plant also produces up to 1,150 tpd of anhydrous ammonia and about 95 MMscfd of CO2. The CO2 is compressed and delivered through a 205-mile pipeline to EnCana Corp.'s oilfields near Weyburn, Saskatechewan, Canada for use in enhanced oil recovery (EOR) [5].
The process of methanation of gases containing CO and hydrogen is well known in the art (see references 1 and 2 below). Typically, the raw syngas exiting the gasifier is first taken to a heat recovery boiler and then to preliminary cleanup to substantially, remove particulates, fines, tars and liquids (if any) along with other trace impurities such as chloride, ammonia and HCN that may be present in the raw gas. The H2/CO ratio of the raw gas is substantially below the necessary minimum ratio of 3/1 typically required for methanation. The desired H2/CO ratio is obtained either by very careful choice and control of the processing conditions, difficult to achieve in continuous processing operations, or by the treatment of the portion of the syngas in a shift conversion reactor to produce a H2/CO ratio substantially in excess of 3/1 and then blending the shifted syngas with the un-shifted portion to produce the desired H2/CO ratio. The mixed stream is then cooled to about 100 F and sent to the AGR unit where CO2 and sulfur compounds are removed by conventional means such as treatment with a suitable physical or chemical solvent-based process, for example Rectisol, Selexol or MDEA technologies. The residual CO2 concentration of the mixed stream prior to entering the methanation reactor is typically maintained at or below 2 mol % to meet required inert specs in final product SNG. Sulfur species in the mixed stream are also removed to substantially under 5 ppm, e.g., to less than about 1 ppm, preferably to less than 0.2 ppm to protect the methanation catalyst from poisoning by such sulfur impurities.
The hydrogen-rich syngas exiting the AGR unit is sent to the methanation reactor that may consist of multiple catalytic fixed beds arranged in series, typically containing high-activity nickel catalyst. Catalytic hydrogenation of CO to produce methane is very exothermic and if not controlled within the reactor, can cause sintering of the catalyst, carbon deposition on the catalyst and/or thermal cracking of product methane to CO and H2. Carbon formation through thermal cracking and/or CO disproportionation in turn has a tendency to foul the catalyst bed. Also, most nickel catalysts active for the methanation reactions tend to deactivate at high temperatures. It is, therefore, important that the gas enters the catalyst bed at the lowest inlet temperature which gives an acceptable initiation reaction rate while still preventing the formation of carbonyl compound which can occur through the reaction of CO with the catalyst at temperatures below proper operating temperatures. To overcome some of these problems caused by overheating or carbonyl formation, extensive recycle streams are used as diluent to absorb some of the exothermic heat evolved. Additional measures for avoiding too high temperature in the reactor include cooling of the catalyst bed or of the reaction gases. For example direct cold gas recycle and internal cooling of the reactor by installing heat exchange surfaces. Most prior art methanation catalysts operate best in the temperature range of 500 F to 900 F.
The exothermic heat evolved during the methanation process is utilized in preheating the feed gas to methanation reactor and in producing steam for process use or power generation. Following methanation, the SNG is compressed, dried and sent to the pipeline.
D. Steam Methane Reforming (SMR)
Steam methane reforming (SMR) is a well known technology for the production of hydrogen from natural gas containing predominantly methane. It is usually carried out by supplying heat to a mixture of steam and natural gas feed while contacting the mixture with a suitable catalyst, usually nickel. In a typical SMR operation natural gas is pretreated to remove sulfur to avoid poisoning of reforming catalyst. This is accomplished by hydrogenation of organic sulfur within a hydrotreater, which converts the organic sulfur to hydrogen sulfide, followed by hydrogen sulfide in a chemisorbent bed, utilizing for example, a zinc oxide sorbent. The desulfurized feed is then mixed with steam and reformed in the reforming reactor to produce syngas containing mainly CO, H2 and CO2. The hot syngas from the reformer is sent for heat recovery to generate high pressure steam and then to a shift conversion reactor in which CO reacts with water vapor at high temperatures over a suitable catalyst to form hydrogen and CO2. Shifted syngas is then taken to hydrogen separation unit such as pressure swing adsorption (PSA) to produce high purity (99.9+ vol %) hydrogen. PSA tail gas is taken to the SMR furnace to burn as fuel. If needed, CO2 present in the syngas stream could be removed using a suitable CO2 removal process prior to taking it to the PSA unit.
A brief description of some of the prior art references is provided below.
U.S. Pat. No. 3,854,895 [1] teaches a process of producing SNG from gasification of carbonaceous feedstock. A method of treating synthesis gas in the methanation reactor to produce SNG is disclosed. SNG contains at least 88 mol % of methane and less than 2 mol % of hydrogen with remainder being CO2 and N2. The synthesis gas produced in the gasifier is divided into two parts to get the required H2 to CO ratio in the feed to methanation reactor. No particular gasifier is discussed.
U.S. Pat. No. 4,199,327 [2] discloses an integrated process in which a non-slagging fixed bed and a slagging type entrained flow gasifiers are used to convert coal to synthesis gas. The synthesis gas is cleaned and used for power, methanol, SNG and chemical feedstock production. Use of two-different type of gasifier in the same process scheme is unique.
U.S. Pat. No. 4,483,691 [3] discloses method for syngas generation in a non-slagging gasifier. The solids and liquid hydrocarbon byproducts present in the raw syngas are removed and subjected to catalytic partial oxidation to produce secondary syngas. Effluent from the catalytic partial oxidation reactor is taken to steam reforming. Acid gases are removed from the syngas and clean syngas is converted to SNG in a methanation reactor.
U.S. Pat. No. 6,676,716 B2 [4] discloses an integrated process scheme in which waste materials are gasified in a fluidized bed gasifier at relatively low temperature to produce syngas. The syngas and the char produced from the gasifier are then used to produce power, F-T liquids, methanol or SNG.
Perry, M. and Eliason, D., “CO2 Recovery and Sequestration at Dakota Gasification Company”, Paper presented at Gasification Technologies Conference, San Francisco, Calif. (October 2004) [5] provides process description for Great Plains SNG plant with CO2 removal and CO2 transportation to an EOR site. Hydrogen and power is not produced in this plant.
Gray, D., Salerno, S, and Tomlinson, G., “Polygeneration of SNG, Hydrogen, Power and Carbon Dioxide from Texas Lignite”, Report Prepared by Mitretek for NETL, U.S. Department of Energy (December 2004) [6] presents integrated concepts for poly-generation of SNG, hydrogen, power and CO2 from gasification of Texas lignite coal. Reliability of hydrogen production and gasification-SMR integration is not addressed.
Miller, C. L., Schmetz, E. and Winslow, J., “Hydrogen from Coal Program—Research Development and Demonstration Plan”, Draft Report Prepared by NETL, U.S. Department of Energy (September 2005) [7] describes various possible pathways for hydrogen production from coal under the U.S. Department of Energy's futuregen program. SMR integration with gasification is mentioned in the context of hydrogen production but no details are provided.